1. Field of the Invention
The invention relates to an apparatus for the detection of wavefronts of electromagnetic radiation, in particular visible and ultraviolet light and soft X-rays, and also to a process for wavefront detection, and a stepper or scanner for microlithography with such an apparatus of the concerned category, for wavefront detection.
2. Technical Field
An apparatus of the concerned category, for wavefront detection, is known from “Phase Measuring Ronchi Test”, by Katsuyuki Omura et al. (pages 523-528 in APPLIED OPTICS, Vol. 27, No. 3, February 1988). In this known apparatus, which is used for testing the imaging quality of a test lens, a monochromatic point light source of a He—Ne laser produces a wavefront, which passes through a collimator and a test lens and thereafter strikes a diffraction grating known as a Ronchi grating. The collimator transforms the wavefront emerging from the point light source into a plane wave striking the test lens, the diffraction grating being situated in the image-side focal plane of the test lens. The diffraction pattern or interferogram produced by the Ronchi grating is recorded by a spatially resolving detector including a TV camera, and is used thereon for the detection of the wavefront leaving the optical system.
DD 0154 239 also discloses an apparatus for wavefront detection of the concerned category termed a shearing interferometer, for the interferometric testing of objectives. In this apparatus for wavefront detection, the diffraction grating is situated outside the focal plane of the objective being tested.
In DE 195 38 747 A1, a grating shear interferometer for wavefront detection of the concerned category is described, in which the light of a plane wave to be investigated falls on two phase gratings, one behind the other, and is then sensed by a CCD camera for wavefront analysis.
A further apparatus for wavefront detection according to the preamble of claim 1 is known from the article “Evaluation of Large Aberrations Using a Lateral-Shear Interferometer Having Variable Shear” by M. P. Rimmer et al., in APPLIED OPTICS, Vol. 14, No. 1, January 1975, pages 142-150. The optical system connected between the wavefront source and the diffraction grating is a curved mirror in this case.
J. E. Pearson et al., in APPLIED OPTICS AND OPTICAL ENGINEERING, Vol. VII, Academic Press, Inc., 1979, Chapter 8, “Adaptive Optical Techniques for Wave-Front Correction”, referred to adaptive optics with hearing interferometers as wavefront detectors and to photolithography as a possible field of application for them.
The article “Dynamic range of Ronchi test with a phase-shifted sinusoidal grating” by K. Hibino et al. in APPLIED OPTICS, Vol. 36, No. 25, September 1997, pages 6178-6189, describes an apparatus for wavefront detection of the concerned category, in which a test lens transforms a monochromatic plane wave and deflects it onto a diffraction grating formed as a sine transmission grating. The wavefronts diffracted by the diffraction grating are collimated by an imaging lens and form, on a rotating matt disk arranged in the focal plane of the imaging lens, a shear interferogram which is recorded by a CCD detector and is then used for testing the test lens.